The present invention relates to an inverted Cassegrain antenna intended for use in the monitoring or tracking modes and which is able to provide a beam which is widened either in the elevation plane (sighting) or in the bearing plane (anticollision) whilst retaining the qualities of a narrow primary beam.
The inverted Cassegrain antenna is known and is described, for example, in U.S. Pat. No. 3,771,160 which relates to an inverted Cassegrain antenna utilizing polarization rotation. The antenna disclosed in this patent comprises a plane auxiliary reflector formed by a plurality of grids of parallel conductive wires and by a metal plate, the plate and the grids of wires being parallel and separated by a dielectric. Such an antenna operates on at least two frequencies, but cannot be utilised in combination with a multiple function monitoring or tracking radar.
In a multifunction radar, it is in fact desirable that the beam transmitted by the antenna have a shape which may be adapted at a given instant to the function for which it is utilised. This has already been produced with simple antennas, by switching of primary sources or by altering the shape of the antenna. However, this means of adapting an antenna for the different functions of a radar does not yield satisfactory results in the case of an inverted Cassegrain antenna. As a matter of fact, the performance factors of the Cassegrain antenna are reduced if the primary sources of this antenna are multiplied or if the parabolic reflector is distorted, which imposes the need to modify the beam focussing device.
An advantageous means of producing a multifunction inverted Cassegrain antenna is to modify the form of the polarizing rotation reflector with which it is equipped, in order to widen the beam in a given direction.
French Pat. No. 2,448,233, (corresponding to U.S. Pat. No. 4,253,100) filed on the 2nd of February 1979, discloses a multifunction inverted Cassegrain antenna comprising a polarizing rotation reflector formed by two or more polarizer-reflector elements articulated in pairs around a hinge orthogonal to the required beam widening direction.
The articulation between the two polarizing reflector elements may be produced in the form of a simple hinge bonded to the rear section of the polarizing rotation reflector. However, this kind of joint produces radioelectric discontinuities in the region of the front surface of the polarizer, as well as in the region of the reflector at the rear side, and a radioelectric mismatching effect in the region of the joint. These discontinuities impair the characteristics of the antenna when the reflector elements are rendered coplanar.
These radioelectric deteriorations which are caused by the noted electric discontinuity are accentuated by the mechanical defects of a hinge of this nature. In effect, a distance of a few millimeters remains between the two sections of the reflector in the coplanar position. Furthermore, the hinge does not cover the whole width of the reflector at the point of the interruption. Consequently, the positioning of the movable element is not of constant accuracy across the whole width of the interruption.
These electrical and mechanical defficiencies cause a rise of the distant secondary lobes for particular sweeps of the antenna.
The present invention makes it possible to remedy the disadvantages referred to above and to maintain a mechanical and radioelectrical continuity in the region of the hinge joining together two elements of the polarizing reflector throughout the antenna sweep and no matter what the angle of inclination of the movable element may be.
The present invention equally has as an object to re-establish the angular phase shift of 180.degree. in the region of the gap present between two elements and to retain the planeity of the polarizer-reflector.